1. Technical Field
The disclosed embodiments relate generally to wireless communication devices and, more specifically, to low noise amplifiers.
2. Background Information
Radio receivers, such as Code Division Multiple Access (CDMA) cellular telephone receivers, generally include amplifiers called low noise amplifiers (LNAs). The CDMA cellular telephone application requires that the LNAs have a very high third-order input intercept point (IIP3) as well as a low noise factor (NF), a high gain, and a low current consumption. There are several linearization techniques that are used to achieve these performance characteristics.
One popular technique involves the use of negative feedback. In a conventional source-degenerated LNA, a source degeneration inductor is used as a feedback circuit. In general, higher linearity may be achieved by increasing the source degeneration inductance and/or by increasing the LNA bias current. Source-degenerated LNAs, however, still suffer from poor linearity due to a second order non-linearity feedback effect. Furthermore, LNAs with larger source degeneration inductances exhibit lower gain and higher noise factor, and the increasing of bias current causes higher power consumption. If bias current is increased too much, then headroom problems are encountered.
A second technique is the derivative superposition (DS) technique. The DS technique uses two or more parallel FETs of different gate widths and gate biases to achieve high linearity and enhanced IIP3 performance. However, the conventional DS method does not significantly increase IIP3 performance at high frequencies due to a second-order nonlinearity contribution to the third-order intermodulation distortion (IMD3).
The modified DS (MDS) technique addresses the second-order nonlinearity contribution. In the modified DS techniques, the magnitude and phase of the third-order nonlinearity contribution to IMD3 is tuned to cancel the second-order non-linearity contribution to IMD3, thereby generating an output current with very low IMD3.
FIG. 1 (Prior Art) is a circuit diagram of an LNA 100 that utilizes the MDS technique. In the MDS circuit of FIG. 1, two FETs 104A and 104B are used. FET 104A is biased in its sub-threshold region (weak inversion) and FET 104B is biased in its saturation region (strong inversion). It is known in the art that the third order nonlinearity contribution component (g3) to IMD3 of a FET changes from positive to negative as the FET operation changes from weak inversion to strong inversion. This means that when the two FETs 104A and 104B are biased at the positive and negative peaks of g3 with equal magnitude, the output currents by the two FETS 104A and 104B are summed and the result is an output current with near zero IMD3. The MDS technique also takes into account the second order nonlinearity contribution component (g2) to IMD3. As illustrated in FIG. 1, a tapped inductor 102 is used such that the magnitude and the phase of g3 is tuned to cancel g2. See Published U.S. Patent Application No. 2005/0176399, published Aug. 11, 2005, for a more detailed explanation of the operation of an LNA that employs the Modified Derivative Superposition (MDS) technique.
FIG. 2 (Prior Art) is a circuit diagram of an LNA 120 that utilizes a variation of the MDS technique. In the MDS circuit of FIG. 2, two FETs 122 and 124 and two inductors 126 and 128 are used. The same general MDS technique of phase cancellation as illustrated in FIG. 1 is used in LNA 120 of FIG. 2 to achieve high linearity. However, by connecting the gate of auxiliary transistor 124 to the source of main transistor 122, LNA 120 of FIG. 2 further improves NF. Moreover, connecting the gate of auxiliary transistor 124 to the source of main transistor 122, allows tuning for input match and linearity to be conducted independently. For further information on this variation of the MDS technique, see: “Highly Linear Low Noise Amplifier”, Texas A&M Master of Science Thesis by Sivakumar Ganesan, pages 1-73, May 2006.
When strong jammer tones are present, an LNA in a CDMA cellular telephone must have high linearity and low distortion. Such high linearity performance is usually achieved using the MDS technique in combination with increased bias current of the LNA. The degree to which the bias current can be increased is, however, limited. On the other hand, when no jammer tones are present, the LNA can have lower linearity and lower power consumption in order to extend battery life of the CDMA cellular telephone.